Lithography is used in micro-fabrication of a semiconductor integrated circuit, MEMS (micro electro mechanical systems) processing, and processing of a surface of a liquid crystal flat panel. In the lithography, pattern exposure is performed on a photosensitive resin layer formed on a substrate to be processed, and the exposed photosensitive resin layer is developed to form an aimed pattern. In the lithography, generally the photosensitive resin layer is irradiated with light through the photomask in which the pattern is drawn. At this point, in order to prevent a flaw caused by a close contact of the photomask with an exposed body, the exposure is usually performed while a gap is provided between the photomask and the exposed body, namely proximity exposure is performed.
However, in the case that the gap is provided, a shape of the pattern is distorted due to a Fresnel diffraction phenomenon even if the pattern having several times size of a wavelength of exposure light is provided on the mask to form the fine pattern having several times size of the wavelength of exposure light. For this reason, there is a problem in that the pattern having the aimed size and shape can hardly be formed on a substrate of the exposed body. Specifically, even if an exposure light having a wavelength of 365 nm is used, only resolution ranging from about 4 μm to about 5 μm can be obtained when the gap of about 50 μm is provided between the mask and the exposed body. The size that is less than or equal to ten times the wavelength is extremely difficult to be formed when the gap of 50 μm or more, which is preferable from the viewpoint of production, is provided.
For example, in the case that the exposure is performed using exposure photomask 103 in which light blocking member 102 is formed on translucent substrate 101 as illustrated in FIG. 29, light 104 transmitted through photomask 103 is diffracted, and the pattern blurs until diffracted light 104 reaches substrate 105 of the exposed body.
However, nowadays a projection exposure method in which a projection lens is provided between photomask 103 and substrate 105 to be processed is occasionally used. In the projection exposure method, because a pattern image is transferred to the exposed body, the resolution can be obtained to a degree of the wavelength of exposure light. However, substantial cost is required for an exposure apparatus because a high-accuracy lens is required for the projection exposure method.
Therefore, there has been proposed a proximity exposure method in which, by devising the mask pattern, the resolution is improved to be able to expose the finer size at low cost.
Before the description of a pattern forming method performed to improve the resolution in the proximity exposure method, a cause of degrading the resolution in the conventional proximity exposure method is described (for example, see Unexamined Japanese Patent Publication No. 2001-100389).
FIGS. 30A to 30C illustrate states of the transfer image when a normal photomask is used in the conventional proximity exposure method. FIG. 30A illustrates desired pattern 110 having a square shape in a planar view, and pattern 110 is used as an opening of the photomask in the normal photomask. FIG. 30B illustrates a light intensity distribution of exposure image 110A when the exposure is performed using the photomask in which pattern 110 in FIG. 30A is formed. FIG. 30C illustrates pattern 110B obtained from exposure image 110A. As illustrated in FIG. 30C, each corner (angle) in formed pattern 110B projects outward, and each of four sides is recessed inward, thereby becoming a distorted shape. This is because the light intensity is increased by light interference in each corner. The distortion is also a cause to largely degrade the resolution in the proximity exposure.
On the other hand, in the projection exposure method, the mask pattern is corrected to improve the resolution of the transfer pattern. FIGS. 31A to 31D are plan views illustrating a typical mask pattern correction in the projection exposure method. FIG. 31A illustrates desired pattern 111 having a square shape in a planar view. FIG. 31B illustrates transfer image 111A expressing the light intensity distribution of the exposure image when the exposure is performed using a mask pattern having the shape identical to that of desired pattern 111 as the opening. FIG. 31C illustrates pattern 111B obtained from transfer image 111A. In the projection exposure method, the distortion is generated such that each corner of square pattern 111 is rounded to become a near-circular shape. As illustrated in FIG. 31C, the proximity exposure method totally differs from the projection exposure method in a typical distortion generated in the mask pattern having the identical shape. The distortion of the transfer image is caused by the interference between light beams transmitted through the mask opening in the proximity exposure method. On the other hand, in the projection exposure method, the distortion of the transfer image is attributed to the fact that the exposure light is transmitted through the opening of the mask having the size to a degree of a resolution limit, which is defined by a lens diameter and a wavelength of the exposure light, and the light that does not contribute to the image formation is generated. Therefore, in the projection exposure method, correction pattern 111C is used to widen an opening area as illustrated in FIG. 31D. In correction pattern 111C, opening 111c is added to the acute-angled corner of the opening pattern through which the exposure light is insufficiently transmitted. Opening 111c is an example of mask pattern correction referred to as serif. A number of mask correction methods are proposed in the projection exposure method (for example, see Unexamined Japanese Patent Publication No. 2001-100389).
As described above, because the proximity exposure method differs from the projection exposure method in the distortion principle of the transfer image, the resolution is not expected to be improved in the proximity exposure method from an analogy of the mask correction that is performed to improve the resolution in the projection exposure method.
The mask pattern correction performed to improve the resolution in response to the pattern distortion phenomenon generated in the proximity exposure method will be described below with reference to FIGS. 32A to 32C (for example, see Unexamined Japanese Patent Publication No. 2008-76940).
As illustrated in FIG. 32A, in photomask 112, second light blocking member 112b that is of a correction pattern is provided in a region including each corner along first light blocking member 112a and is provided inside opening 112c surrounded by first light blocking member 112a. Therefore, the distortion of the pattern shape by increasing the light intensity due to the light interference in each corner is prevented.
FIG. 32B is a view illustrating an effect of second light blocking member 112b of the correction pattern, and illustrating the light intensity distribution of the exposure image when the exposure is performed using photomask 112 in FIG. 32A. As can be seen from FIG. 32B, the portion in which the light intensity is increased by the light interference in each corner is eliminated compared with exposure image 111A in FIG. 30B. FIG. 32C illustrates pattern 112B obtained from exposure image 112A. Referring to
FIG. 32C, although a high light-intensity portion caused by new interference emerges near a central portion of pattern 112A, the high light-intensity portion has a small influence on the shape of pattern 112A because the high light-intensity portion is located near the central portion of pattern 112A.